Certain ceramic processes require alumina of a specified range of particle size as a primary raw material. This invention provides a process for obtaining the particle size distribution required to produce satisfactory product in a more efficient and economical manner.
Classification of alumina, which is separation according to particle size, is presently done by gravity sedimentation from an aqueous dispersion. Typically, the alumina is placed in water to which hydrochloric acid has been added. In one recipe, the weight percentage of alumina is initially 15% and the pH is 4.0. The hydrochloric acid stabilizes the dispersion of the alumina. If the pH is inadvertently changed by more than about 0.5 units either way, the alumina flocculates rapidly.
A properly prepared alumina dispersion is quite stable, as the individual alumina particles are held apart by electrostatic forces and do not form large agglomerates which settle quickly. In such a stabilized dispersion, the particles settle very slowly under the influence of gravity with the larger particles settling more rapidly. This is the basis for the classification process.